@Tella it may be possible to have antibody screening tests for mono (EBV) But I am unsure how they work....It may show that you had it at some time, or that it has reactivated (?) Others will know more about it, and hopefully, might comment.
Mono does have distinctive symptoms which might set it apart from the regular "flu" or something similar:
Fatigue.
Sore throat, perhaps misdiagnosed as strep throat, that doesn't get better after treatment with antibiotics.
Fever.
Swollen lymph nodes in your neck and armpits.
Swollen tonsils.
Headache.
Skin rash.
Soft, swollen spleen.
But of course, if you had an atypical form, it might be hard to know if you had it or not. I knew someone who did have it, and she took some time to feel better, though did make a full recovery. I seem to recall she did have some low times for at least a year afterwards. She was very young (about 17 when she caught it) That was in the early 80s.
It's difficult to tell what infection you might have had, if it was an atypical form, and the symptoms didn't fit in the profile.
I was the same with my peculiar "flu". It didn't fit into any profile of any viral infection whatsoever ! And believe me, I exhausted the list of viral infections !